This invention relates to halogen lamps of the type that are used in vehicle headlights and to manufacturing methods for making such lamps.
Halogen filament lamps generally comprise tubular vitreous envelopes enclosing a filament which is surrounded by an inert halogen gas. Such lamps are used in headlight systems for vehicles and have replaced traditional incandescent lamps which have lower light output with a higher energy consumption. Halogen incandescent lamps generally utilize a tungsten filament which is supported and connected to electrical lead wires which supply the filament with current and cause the filament to produce incandescent light. The presence of a halogen gas within the vitreous envelope allows for the recycling of the tungsten atoms which are released into the surrounding volume as the filament is heated to increase the life of a lamp.
A more recent development for increasing the efficiency of such halogen lamps has been to include a coating or filter which transmits visible light radiation but reflects infrared radiation back to the filament thereby decreasing the amount of electrical power used by the lamp without a significant decrease in the amount of visible light output. Such coatings or filters are known in the art and maybe found for example in U.S. Pat. Nos. 4,663,557 and 4,701,663.
When such infrared reflective coatings are utilized, it is necessary that the filament be centered or aligned along the optical axis of the vitreous tube for the coating to effectively reflect the infrared radiation back onto the filament.
U.S. Pat. No. 4,942,331 to Bergman et al. discloses a double ended HIR (halogen infrared) filament lamp including a quartz glass tubular envelope having an infrared reflective coating and enclosing an axially-oriented tungsten filament that is connected to a molybdenum foil inlead. The inlead is connected to the filament utilizing plasma or laser welding to join the ends of the filaments and inleads. The inleads include spuds which generally comprise a refractory metal wire in the shape of a circular ring that is connected to the filament and allows for positioning of the filament within a central portion of the quartz tube. The lamp manufacturing technique of the U.S. Pat. No. 4,942,331 patent utilizes plasma or laser welding operations to connect the spud with the filament which is enclosed in a high melt temperature quartz glass. This combination of quartz glass tubing, spuds, and molybdenum foil inleads can be difficult and expensive to manufacture.
In non-HIR automotive headlamp applications where relative positioning of multiple filaments is needed, it is known to form a single-ended halogen lamp using a positioning device that is inserted into a first end of a glass tube to hold high and low beam filaments in a set position during sealing of a second end of the tube. See, for example, U.S. Pat. No. 4,305,632 to de La Chapelle. For transverse mounted filaments, the positioning device includes a pair of transverse slots in its lower end. The slots are spaced apart by a separator having a width suitable for maintaining the desired spacing of the filaments. For axially-oriented filaments, the patent states that the positioning device would have longitudinal grooves or holes to contain the filaments during the press sealing operation. The positioning devices disclosed in this patent are used to set the position of one filament relative to another within a generally cylindrical glass envelope that does not have an infrared reflective coating and that does not have a spherical or ellipsoidal shape suitable for use with such coatings.
There is therefore a need in the art for producing a cost effective halogen lamp having an enlarged envelope that includes an infrared coating and a filament that is precisely centered within the envelope. It is therefore a general object of the present invention to provide a method of producing a single ended tungsten halogen lamp having an enlarged envelope with an infrared reflective coating and a filament radially centered within the envelope.
In accordance with one aspect of the present invention, there is provided a method of making a halogen lamp having an axially-oriented filament. The method includes the steps of:
(a) providing a glass tube having first and second ends and an enlarged section located between the first and second ends;
(b) providing a filament assembly that includes a filament supported on one or more lead wires;
(c) inserting the filament assembly into the first end of the glass tube such that the filament is oriented axially within the enlarged section of the glass tube;
(d) centering the filament within the enlarged section of the glass bulb using a mandrel alignment tool that is inserted into the second end of the glass tube;
(e) sealing the first end of the glass tube around the filament assembly;
(f) removing the mandrel alignment tool from the glass tube;
(g) filling the glass tube with a halogen gas; and
(h) sealing the glass tube at a location between the enlarged section and the second end.
Preferably, the mandrel alignment tool has a base with tip that extends down from the base and into the center of the filament which is preferably in the conventional form of a coil. At least a section of the base has an outer diameter that is the same as the inner diameter of the glass tube so that when that section of the base is inserted into the glass tube, the tip is centered radially and the mandrel alignment tool is restricted from any radial movement. The enlarged section of the glass tube preferably has a spherical or ellipsoidal shape and is coated on its exterior surface with an infrared reflective material.
In accordance with another aspect of the present invention, there is provided a single-ended halogen lamp that can be manufactured according to the inventive method disclosed herein. The lamp includes a glass envelope having first and second sealed ends and a spherical or ellipsoidal region located between the first and second ends. A pair of leads extend through the first sealed end from an exterior, exposed location to an interior located within the glass envelope. A filament is electrically connected to the leads, with the filament being oriented along an axis extending between the first and second ends and being radially centered within the glass envelope. A halogen gas fill is contained within the glass envelope and an infrared reflective coating is disposed on a surface of the spherical or ellipsoidal region to reflect infrared light back onto the filament for increased efficiency.
Preferably, the filament is a coiled tungsten filament axially oriented within the glass envelope and the region is ellipsoidal in shape with its two foci each located at the filament proximate an opposite end of the filament.